design landing page with title, about section/page, and more visible link to documentation

[ngehlenborg]

@sehilyi @dominikglodzikhms, related to #105:

• [x] add title of the tool ("Chromoscope") to homepage/overview. The sample count should be shown below the dropdown with the cohort selection or somewhere to the right of the screen.
• [x] add a link to the GitHub repo that includes the word "GitHub" in addition to an icon
• [x] add a link to documentation that includes the word "Docs" or better "Documentation" in addition to the icon
• [x] add an About section/popup that also lists paper/preprint citation information once available